My theory: Blackbeard is .
You see, before Blackbeard became Blackbeard, he made a deal with the man on the moon, otherwise known as The Devil. The true Blackbeard is not the Blackbeard we know today. The real Blackbeard, Marshall D. Teach, was a kind and honest man who was compassionate to all and respected everyone's differences. However, he had a dream. And that dream was to become a professional mahjong player.
That's right. He set out after college to look for job opportunities in the mahjong field, but soon after he graduated from his senior year, a tragedy struck. Thatch was killed! He was hit by an oncoming train. You see, Marshall was actually delusional and thought that everyone of his friends were in a pirate crew with him.
Marshall spent years grieving. Finally, he turned to his confidant, the full moon. "Why, oh moon, must thee forsaken I?" And the moon said unto him; "Because it was your destiny." Marshall responded; "What can I do to achieve happiness?" And the moon returned unto him; "Sacrifice your soul."
Marshall did what he was told. He sacrificed his soul to the moon goddess! The moon goddess, Koromo Amae, descended from the heavens. She said unto him; "What is it you desire?" And Marshall responded unto her; "I wish to achieve happiness!"
What Marshall did not know was that Amae was like a genie. She would grant your wish, but with an evil twist. "Alright...but on one condition," she said. "I must morph my soul into your body!"
Marshall was hesitant but complied. "Yes, take me!" And so Koromo Amae absolved herself into him.
This caused Marshall to become mean; meaner than he'd ever been. He took on an alias, "Blackbeard", and betrayed his friends before forming his own very real pirate crew, the Blackbeard Pirates. He took Thatch's former Devil Fruit and ate it himself! As resignment, his friends fired him from chess club!
Now Blackbeard and his crew roam the seven seas, gathering as much of a pirate army of robot overlords that they can find and eating as many Devil Fruits as they can obtain. You never know just exactly who Blackbeard is out there. He could be your little old lady neighbor. He could be your best friend. Or he could be...you!